The Adaptation of Unmodified Strains of Yellow Fever Virus to Cultivation in Vitro by Hugh

Using a technique similar to that devised by Rivers (1) for the cultivation of vaccinia virus, Haagen and Tbeiler (2) reported successful cultivation of a strain of yellow fever virus which had been greatly modified through many mouse passages by intracerebral inoculation. The medium consisted of chick embryo tissue and Tyrode solution containing 10 per cent of normal monkey serum. However, efforts to cultivate unmodified strains of yellow fever virus failed (3). Mter many attempts, Lloyd, Theiler, and Ricci (4) finally succeeded in establishing the unmodified Asibi strain in tissue cultures in which mouse embryo was used as the tissue component of the medium in place of chick embryo tissues. This strain of virus has now been • under continuous cultivation in this laboratory for over 3 years. The Asibi strain of yellow fever virus is the most virulent both in viscerotropic and in neurotropic properties so far isolated. Through continued cultivation in vitro, it has lost much of its pathogenicity (4, 5) and is used at present for human vaccination against yellow fever (6). However, as practically nothing is known regarding the permanency of the changes that take place in the virulence of the virus during prolonged in vitro cultivation, it seemed desirable to adapt strains of less initial pathogenicity to cultivation in tissue cultures. Many of the strains of virus recently isolated from human cases of yellow fever are of relatively low virulence and only moderately patho-genic for rhesus monkeys. It was considered probable that these strains would lose their virulence in tissue cultures within a shorter time and with less danger of reversibility than the highly virulent Asibi strain.